I believe he only stops dropping back because he sees Carr turn to throw, he puts his foot in the ground and drives forward to go after the ball. You can see here Carr is midway through his throwing motion and Shaheed hasn’t gotten past him.
I still don’t understand what Carr saw pre-snap, both those receivers are essentially doubled and the DB’s were too far back to turn and run with the receivers before the ball gets thrown to Kamara (if that was the plan).